Add perp and perp_dot methods for Vec2

Author: jdahlstrom

core/src/math/vec.rs

@@ -273,6 +273,23 @@ impl<B> Vec2<B> {
     pub fn to_vec3(self) -> Vec3<B> {
         vec3(self.x(), self.y(), 0.0)
     }
+
+    /// Returns `self` rotated 90° counter-clockwise.
+    pub fn perp(self) -> Self {
+        vec2(-self.y(), self.x())
+    }
+
+    /// Returns the "perpendicular dot product" of `self` and `other`.
+    ///
+    /// This operation is also called a "2D cross product". Like its 3D analog,
+    /// it satisfies the identity
+    ///
+    /// **a**<sup>⟂</sup> · **b** = |**a**| |**b**| sin *θ*,
+    ///
+    /// where θ is the (signed) angle between **a** and **b**.
+    pub fn perp_dot(self, other: Self) -> f32 {
+        self.perp().dot(&other)
+    }
 }
 
 impl<R, Sc, B> Vector<R, Real<3, B>>
@@ -775,6 +792,23 @@ mod tests {
                 vec4(1.0, -2.0, 4.0, -3.0)
             );
         }
+
+        #[test]
+        fn perp() {
+            assert_eq!(Vec2::<()>::zero().perp(), Vec2::zero());
+            assert_eq!(Vec2::<()>::X.perp(), Vec2::Y);
+            assert_eq!(vec2(-0.2, -1.5).perp(), vec2(1.5, -0.2));
+        }
+
+        #[test]
+        fn perp_dot() {
+            const X: Vec2 = Vec2::X;
+            const Y: Vec2 = Vec2::Y;
+
+            assert_eq!(X.perp_dot(X), 0.0);
+            assert_eq!(X.perp_dot(Y), 1.0);
+            assert_eq!((2.0 * Y).perp_dot(3.0 * X), -6.0);
+        }
     }
 
     mod i32 {